Usually the diameter of the chandelier is equal in inches to the sum of the 2 sides of the room (expressed in feet). So your room's sides are 21 feet in total, which points to a 21" diameter chandelier, give or take a few inches. But it also depends on the fixture. The Wetumka you are considering isn't very bulky, so you probably could go with the 25" 5-light one. I think the 7-light would be overkill for your room.